Use GitHub Actions to test on MS Windows

[cjmayo]

Includes adapted file tests, partially from #127 by Petr Dlouhý.

If anyone using Windows wants to help:

• A couple of tests fail, the reasons need investigating
• Testing takes too long, some tests appear to be waiting for something

[cjmayo]

The failures are:

 tests\checker\test_file.py:85: in test_markdown
    self.file_test("file.markdown", confargs=confargs)
tests\checker\__init__.py:262: in file_test
    self.fail(msg)

 tests\checker\test_loginurl.py:56: in test_loginurl
    self.assertEqual(cookies["login"], "test_user")
.tox\py\lib\site-packages\requests\cookies.py:328: in __getitem__
    return self._find_no_duplicates(name)
.tox\py\lib\site-packages\requests\cookies.py:399: in _find_no_duplicates
    raise KeyError('name=%r, domain=%r, path=%r' % (name, domain, path))
E   KeyError: "name='login', domain=None, path=None"
------------------------------ Captured log call ------------------------------
WARNING  urllib3.connectionpool:connectionpool.py:452 Failed to parse headers (url=http://localhost:50548/tests/checker/cgi-bin/input2cookies.py): Unknown, unparsed data: 'Set-Cookie: login=test_user\r\r\nSet-Cookie: password=test_password\r\n'
Traceback (most recent call last):
  File "d:\a\linkchecker\linkchecker\.tox\py\lib\site-packages\urllib3\connectionpool.py", line 446, in _make_request
    assert_header_parsing(httplib_response.msg)
  File "d:\a\linkchecker\linkchecker\.tox\py\lib\site-packages\urllib3\util\response.py", line 71, in assert_header_parsing
    raise HeaderParsingError(defects=defects, unparsed_data=unparsed_data)
urllib3.exceptions.HeaderParsingError: Unknown, unparsed data: 'Set-Cookie: login=test_user\r\r\nSet-Cookie: password=test_password\r\n'